In recent years, so-called silicon photonics technology has been actively developed in which an optical-signal transmission line made of silicon (Si) is formed on a part of a semiconductor substrate. The semiconductor substrate is implemented as an optical communication module by integrating an optical device and an electron device, respectively including the transmission line.
Such a semiconductor device may include an optical-signal transmission line that has an optical waveguide composed of a semiconductor layer formed on a substrate with an insulating layer interposed between the semiconductor layer and the substrate, and has an insulating film formed over the optical waveguide on the insulating layer. The insulating layer and the insulating film are made of materials having a lower index of refraction than the semiconductor layer. Thus, the semiconductor layer acts as a core layer while the insulating layer and the insulating film act as clad layers. A semiconductor device including a silicon optical waveguide formed on a part of a semiconductor substrate is advantageous in that power consumption is quite low and an optical communication module can be implemented in a small size by forming a silicon integrated circuit and the optical waveguide on the same silicon substrate.
In Andy Eu-Jin Lim et al., “Review of Silicon Photonics Foundry Efforts” IEEE Journal of selected Topics in Quantum Electronics Vol. 20. No. 4 July/August 2014, a semiconductor device is described in which a heater is formed above an optical waveguide formed on a part of a semiconductor substrate, the heater including a titanium nitride (TiN) film on an insulating film covering the optical waveguide.